Grid-connected inverters are commonly used for various applications. One of the main applications for grid-connected inverters is in distributed generation (DGs) of power. In DGs, grid-connected inverters are the interface between the utility grid and the power conditioning systems for other energy sources (e.g., solar, wind, etc.). DC/AC inverters usually operate under hard-switching where neither the voltage nor the current of the power switches is zero during the switching transitions. The power semiconductors used in DC/AC inverters are switched under very high voltage at the intermediate DC-bus (usually more than 400V). Switching losses of the power semiconductors in such inverters significantly contribute to the overall losses of the power conditioning system. In particular, reverse recovery losses of the power semiconductors' body diodes are inevitable for such topologies. Because of this, the switching frequency of the inverter is very limited, usually in the range of 10-20 kHz. Because of strict regulatory standards, high quality current needs to be injected into the utility grid from such inverters. To accomplish this, such inverters require large filters at their outputs.
Another issue with low switching frequencies is that such low frequencies create a high amount of current ripple across the inverter output inductor. This current ripple not only increases the core losses of the inductor but it also increases the inductor's high frequency copper losses. In addition to this, reducing the DC-bus voltage creates a significant amount of conduction and emission EMI noise. The high amount of conduction and EMI noise may affect the operation of the control system and may highly degrade the system's reliability.
From all of the above, it can be gathered that hard-switching limits the switching frequency of the converter and imposes a substantial compromise in the design of the output filter and in the overall performance of power conditioning systems.
To address the above issues, soft-switching may be used. There are many different soft-switching techniques reported in the literature. However, these techniques generally require many extra active/passive circuits. In particular, extra active circuits highly deteriorate the reliability of the system due to the additional complexity imposed by the active components. Also, the effectiveness of these techniques is questionable. Some studies have shown that some soft-switching techniques may add more losses to the converter and, consequently, greatly offset their advantages. Because of this, most industrial products use the more conventional hard-switching inverters with large filters in order to ensure a reliable power conditioning system. Even though the system's performance is highly compromised with hard-switching and bulky lossy filters, industrial decision makers prefer to use a reliable, well-known solution for the inverter.
Auxiliary circuits have previously been used to provide soft-switching conditions for the power semiconductors for voltage source inverters. Such soft-switching circuits usually use a combination of active and passive circuits in order to provide soft-switching conditions. However, generally, active circuits increase the complexity of the power circuit and reduce the reliability of the systems. In addition to this issue, losses related to auxiliary circuits usually highly offset the advantages of the soft-switching and thereby compromise the converter performance. As well, it is usual that auxiliary circuits include a resonant circuit with a very high amount of peak current/voltage. There are, therefore, significant amounts of losses attributed to auxiliary circuits. As well, it should be clear that passive components should be able to withstand the high amounts of currents and voltages during switching transitions. FIG. 1 shows an exemplary arrangement for such auxiliary circuits (prior art).
In DC/AC inverters with hard switching and low switching frequency, the current ripple across the output inductor creates a significant amount of power losses. The impact of this current ripple on the power losses is two-fold. First, the current ripple produces core losses and, second, the current ripple also increases the high frequency ohmic losses of the converter. Usually, magnetic wires are used to wind the output inductors and these magnetic wires show a significant resistance to the high frequency currents. In conventional designs, the current ripple is usually kept very small so that only a negligible high frequency current ripple is produced. As well, in conventional designs, the current ripple is usually kept very small so that only a small amount of the attributed high frequency ohmic losses are produced. Unfortunately, another restriction is that, in high power density designs, even though the magnetic wires resist the high frequency currents, the inductors cannot be very bulky.
In order to reduce the current ripple injected to the grid and to thereby improve the quality of the grid current, an LCL-filter is usually used at the output of the inverter. LCL-filters provide higher attenuation for high frequency harmonics compared to conventional L-filters. FIG. 2 shows an exemplary arrangement for the DC/AC inverter with LCL-filters (Prior Art).
While LCL-filters can be useful, there are some difficulties created by LCL-filters in DC/AC inverters. The first problem is that LCL-filters provide a path for the grid even when the inverter is off. In particular, in grid-connected inverters, there is a constant current drained from the grid. Also, the LCL-filter can resonate with other components (e.g., RLC loads) connected at the Point of Common Coupling (PCC) through this current path in micro-grid applications. Since the inverter has no control of this resonance, the resonance current can be substantial and can harm different components connected to the point of common coupling. It should be noted that this resonance is the one between the external components and the current path including L2 and C inside the inverter, not the resonance created by the LCL-filter which can be damped by the control system.
FIG. 3 shows an exemplary arrangement of a system with an LCL-filter and which illustrates the current paths provided by the LCL-filters. Another difficulty with the LCL-filter is the physical space it occupies in the circuit as it requires two inductors. This difficulty is especially relevant in low profile applications where LCL-filters can take up a significant real estate of the board as they require two separate inductors. It should be noted that, even though LCL-filters require two smaller inductors when compared to conventional L-filters, LCL-filters usually take up more space since they require two separate magnetic cores.
What is needed is a simple and practical solution which provides soft-switching for the power semiconductors and which, simultaneously, does not compromise system reliability. Also, the high frequency current component of the output inductor is preferably limited in order to provide a high power density converter with superior efficiency. It is therefore preferable to only use passive components for soft-switching the power semiconductors and to also reduce the high frequency component of the inverter output current. Also, another preference is to optimize (or minimize) any losses associated with the extra passive components. Such preferred optimized losses due to the extra components should not offset the advantages of soft-switching.